Loose wrists are one of the most misunderstood attributes associated to playing the piano. On the contrary, when performing you need to be in complete control of your forearms, which of course include your wrists. I was taught this by Keuorilisky, who fingered for Horowitz on occasion, and Voorhies, who helped me win many competitions and was one of the really good teachers of our time. The most famous piano performer who believed in loose wrists was Liberachi, so having said that, you be the judge, Liberachi....Horowitz...??
Did you ever know that physical fitness has a lot to do with keyboard performance? While training for competition I used to do pushups on my fingers to strengthen my forearms which accomplished two things. One, it enabled me to perform grueling compositions like Chopin's Etude in C minor without tiring, and two, enabled me to perform at volume levels at both extremes, whether loud enough to fill a concert hall, or soft enough not to wake a sleeping child.